A discrete number of cell surface antigens and secreted antigens are known to regulate leukocyte function. In particular, these antigens govern leukocyte activation, proliferation, survival, cell adhesion and migration, effector function, and the like. Among those antigens that have been shown to regulate leukocyte function are members of the scavenger receptor cysteine-rich ("SRCR") domain-containing protein family. Members of this protein family have conserved sequence motifs that are characterized by short, disulfide-stabilized domains.
The SRCR domain was initially recognized during the analysis of the structure of the type I macrophage scavenger receptor in which a motif of approximately 101 amino acid residues was identified. The SRCR domain defines an ancient, highly conserved family of cysteine-rich proteins and is present in CD5 and CD6 molecules (Freeman et al. (1990), Proc. Natl. Acad. Sci USA 87:8810).
The SRCR family of proteins can be divided into two groups, designated group A and group B. The groups may be distinguished primarily by the presence of 6 (group A) or 8 (group B) positionally conserved cysteine residues within each SRCR domain, however, proteins having 6 such residues within each SRCR domains have also been characterized as group B proteins at least in part by the presence of cysteine residues at the C.sup.1 and C.sup.4 positions (Resnick et al. (1994) Trends Biochem. Sci. 19:5-8). Independent SRCR consensus sequences for groups A and B, as well as a combined consensus sequence, have been identified (see, Resnick et al. (1994), supra). Group B includes the cell surface proteins CD5 (Jones et al. (1986) Nature 323:346-349) and CD6 (Aruffo et al. (1991) J. Exp. Med. 174:949-952), which are predominantly expressed by thymocytes, mature T cells and a subset of B cells, WC1 (Wijngaard et al. (1992) J. Immunol. 149:3273-3277; Wijngaard et al. (1994) J. Immunol. 152:3476-3482), which is expressed by .gamma..delta. T cells in cattle, and M130 (Law et al. (1993) Eur. J. Immunol. 23:2320-2325), which is expressed by activated monocytes.
Monoclonal antibody (mAb) crosslinking studies suggest that both CD5 and CD6 can function as accessory molecules capable of modulating T cell activation (Gangemi et al. (1989) J. Immunol. 143:2439-2447; Ledbetter et al. (1985) J. Immunol. 135:2331-2336). This role of CD5 and CD6 in the regulation of T cell function is supported by the finding that, following T cell activation, tyrosine residues in the cytoplasmic domain of these two proteins are transiently phosphorylated. This would provide a molecular mechanism by which the cytoplasmic domains of both CD5 and CD6 can interact with intracellular SH2 containing proteins involved in signal transduction (Raab et al. (1994) Mol. Cell. Biol. 14:2862-2870). Furthermore, phenotypic analysis of a CD5-deficient murine strain showed that its T cells are hyper-responsive to stimulation (Tarakhovsky et al. (1994) Eur. J. Immunol. 24:1678-1684; Tarakhovsky et al. (1995) Science 269:535-537), suggesting that CD5 expression is required for the normal regulation of T cell receptor-mediated T cell activation. In addition, comparison of anti-immunoglobulin M-induced growth responses in B-1 and B-2 cells from wild-type or CD5-deficient mice indicated that CD5 acts as a negative regulator of membrane immunoglobulin M-mediated signaling in B-1a cells.
Bikah et al. (1996) Science 274:1906-1909. These authors suggested that certain autoimmune states may be due to defects in CD5-mediated negative regulation of membrane IgM signaling.
CD5 and CD6 are structurally the most closely related members of the group B SRCR family of proteins (Resnick et al., supra). They are both type I membrane proteins whose extracellular region is composed of three SRCR-like domains each containing 8 cysteine residues which are thought to form intrachain disulfide bonds. The extracellular domains of CD5 and CD6 are anchored to the cell membrane via a hydrophobic transmembrane domain and a long cytoplasmic domain. It has been reported that CD5 binds to the B cell antigen CD72 and to CDSL, an antigen which is transiently expressed by activated B cells which has yet to be fully characterized. CD6 has been shown to bind to the leukocyte activation antigen, activated leukocyte cell adhesion molecule ("ALCAM"). Unlike CD5 and CD6, which are closely related, CD72 and ALCAM are not homologous. CD72 is a type II membrane protein which is homologous to the C-type lectins, however, a lectin activity for CD72 has not been reported. ALCAM is a type I membrane protein whose extracellular region is composed of five Ig-like domains (Bowen et al. (1995) J. Exp. Med. 181:2213-2220). The regions of CD5 and CD72 involved in their interaction have not been identified. Studies with truncated forms of both CD6 and ALCAM have shown that the interaction between these two proteins is primarily mediated by the membrane proximal SRCR domain of CD6 and the amino terminal Ig-like domain of ALCAM (Whitney et al. (1995) J. Biol. Chem. 270:18187-18190; Bowen et al. (1996) J. Biol. Chem. 271:17390-17396).
The identification of a novel molecule involved in leukocyte function provides a new target for monitoring immunoregulatory function and for therapeutic intervention therewith.